DESCRIPTION (Adapted from the Investigator's Abstract) The objective of this proposal is to determine the function of RAD52 in mammalian somatic cells. Extensive genetic evidence implicates the RAD52 gene in a recombinational DNA repair pathway in the yeast Saccharomyces cerevisiae. Mutations that inactivate this gene render yeast strains exquisitely sensitive to DNA damaging agents, and dramatically reduce the frequency of nearly all forms of cellular homologous recombination. Murine and human homologs of this gene have been identified, and shown to be expressed in somatic tissue. The principal investigator and others have shown that overexpression of either human or yeast alleles of this gene in mammalian system renders the host cells resistant to DNA damaging agents, while simultaneously enhancing the frequency of cellular homologous recombination. In addition, the PI have determined that overexpression of human antisense RAD52 mRNA renders human fibrosarcoma cells sensitive to a number of physical and chemical DNA damaging agents. The PI believes that these observations support the hypothesis that a RAD52 recombinational repair pathway functions in mammalian somatic cells. One of the major objectives of this proposal is to test this hypothesis by creating mammalian somatic cells that lack a functional RAD52 gene. Similar RAD52-knockout experiments will be performed in cell lines deficient in DNA-dependent protein kinase activity. The PI suggests that these latter experiments will permit him to evaluate the relative importance of the recombinational and end-joining repair pathways in mammalian cells, and allow him to test the hypothesis that these two apparently distinct repair pathways genetically interact. In addition, the PI intend to identify and clone mammalian Rad52-binding proteins. The objective of this strategy is to both gain insight into the exact function of the Rad52 protein, as well as to further dissect the RAD52-dependent recombinational repair process in mammalian cells. There has been a great deal of interest in the role of DNA repair in human cancer of late. Recent developments have shown that the link between deficient DNA repair and cancer is more extensive than had been previously believed. It has also become clear that tumor cells may utilize enhanced DNA repair activity to evade the cytotoxic effects of chemotherapeutic agents. A greater understanding of the function of RAD52-dependent DNA repair in mammalian somatic cells is therefore likely to have important implications for the fields of carcinogenesis and anti-cancer therapy.